1. Field of the Invention
The present invention relates to a control device for a motor employed for an electric vehicle and, more particularly, to a motor control device which is designed to prevent the deterioration of a storage battery or the like by preventing an excessive increase in the regenerative braking amount of the motor.
2. Description of the Related Art
FIG. 6 is a block diagram showing a motor control device for an electric vehicle. The motor control device for the electric vehicle is equipped with a motor 1, which is operatively connected to driving wheels (not shown), and a driver circuit 2 which is connected to the motor 1 to drive the motor 1. The motor control device is further provided with a controller 3 for controlling the motor via the driver circuit 2, a storage battery 4 for supplying power to the motor 1, the driver circuit 2 and the controller 3, a brake stroke sensor 5 which is provided on an operating section of each brake (not shown) for detecting an the operating amount of the corresponding brake and supplying the detected operating amount of the brake to the controller 3, and a revolution detector 6 which is provided on the motor 1 for detecting the number of revolutions (i.e., rpm) of the motor 1 and supplying the detected number of revolutions to the controller 3.
FIG. 7 shows the relationship between regenerative power and a maximum regenerative torque with respect to the number of revolutions per minute of the motor 1. As it is obvious from the chart, the relationship between the regenerative power and the maximum regenerative torque satisfies the expression given below:
Regenerative power.varies.Max. regenerative torque.times.Number of revolutions (rpm)
Decelerating a vehicle at a fixed deceleration value gives good deceleration feeling without causing a shock to a driver and/or passenger. To achieve this, it is necessary to regenerate the motor of the electric vehicle at a fixed torque, so the regenerative power is apropriately set as shown in a diagram of FIG. 8. According to FIG. 8, the regenerative torque becomes constant when a set rpm N0 or more is reached. Further, in an actual vehicle, regenerating the motor so far as the rpm becomes zero causes a shock or the like immediately before reverse rotation or stoppage of the motor 1. For this reason, there are some cases where the regenerative torque is reduced to zero at a certain rpm, as shown in FIG. 9.
As stated above, FIG. 8 and FIG. 9 show an example of the regenerative torque corresponding to the rpm of the motor 1. FIG. 10 shows the regenerative torque (brake regenerative torque) of the motor 1 with respect to the output value of the brake stroke sensor 5. FIG. 11 shows the relationship between the regenerative torque generated by a wheel brake and the rpm. The regenerative power shown in FIG. 8 and FIG. 9 corresponds to the engine brake of a gasoline-powered automobile and the regenerative power is so set that it is applied when an accelerator pedal, which is not shown, is fully released. The regenerative braking amount, which is the regenerative torque of the vehicle, is designed to be equal to the sum of the regenerative torque generated by the aforesaid wheel brake and the regenerative torque corresponding to the engine brake.
Hence, as the brake operating amount or the rpm of the motor 1 increases, the regenerative braking amount accordingly increases, causing the rechargeable storage battery 4 to be charged with an excessively large magnitude of current. At this time, the voltage of the storage battery 4 increases with the charging current and such an increase is significant when the storage battery is fully charged, as illustrated in FIG. 12.
The above-mentioned motor control device for an electric vehicle is incapable of suppressing the regenerative braking amount of the motor, presenting the following problems. As described above, in certain cases, the regenerative braking amount considerably increases and if the voltage of the storage battery 4 increases to exceed a maximum permissible voltage level, then the controller 3, the driver circuit 2 and other component units will be damaged. Likewise, if the storage battery charging current increases to exceed a maximum permissible current level, then the storage battery 4 may be deteriorated or broken.